Modulation of the resorptive activity of rat osteoclasts by small changes in extracellular pH near the physiological range.

We investigated the effect of small shifts in extracellular pH on the resorptive activity of rat osteoclasts in vitro. Osteoclast-containing mixed bone cell populations disaggregated from neonatal rat long bones were cultured for 26 h at low density on bovine bone disks in HCO-3/CO2-buffered medium modified by the addition of small amounts of protons as HCl (2.5-15.0 mEq/L) or hydroxyl ions as NaOH (2.5-10.0 mEq/L). Little or no bone resorption occurred when ambient pH was above about 7.30; at pH 7.00, osteoclasts exhibited near-maximal activity. The greatest changes were associated with a pH difference of as little as 0.10 unit, between pH 7.25 and pH 7.15. Over this narrow range, the average number of resorption pits formed on each bone wafer increased sixfold. The steep sigmoidal pH response curve indicates that very slight alterations in ambient hydrogen ion concentration can effectively "switch on" or "switch off" rat osteoclasts in vitro.